I Am A 35 Year Old Female Who Has Been

I am a 35 year old female who has been experiencing periodic vaginal bleeding in between periods, (I call a second period), where it starts out feeling bloated, like I have gas or I ate something I shouldn't have and low abdominal pain, especially the right side. it feels different than my normal cramping I have during my period. I had it checked out about 1 year ago, had ultrasounds, pap smear, everything came out normal. It comes in cycles and comes during spring and during this time of the year. Then sometimes it stops. But now it is more frequent. I haven't had a sexual partner in 2 years and I am not on birth control. Could it be hormonal, or something more serious?